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5
are also known to create currents in
protective conductors. Faulty and
interconnected equipment can also
create protective conductor currents.
In reality, considering that many
modern "intelligent" electrical items
such as washing machines,
refrigerators, A/V equipment, etc.,
have embedded computer-processing,
protective conductor currents are very
common and not just a problem for
industrial or commercial applications.
Essentially, the requirements of
543.7 are related to the levels of
protective conductor current :
543.7
by Mark Coles
This article looks at 543.7 Earthing requirements for the
installation of equipment having high protective
conductor currents - a new group of regulations included
in BS 7671:2008 Requirements for Electrical
Installations, The IEE Wiring Regulations 17th Edition.
543.7 is a new group of regulations
which replaces the now deleted
Section 607 Earthing requirements for
the installation of equipment having
high protective conductor currents.
Section 607 was previously a Special
Location in Part 6 of BS 7671 but, as
this type of installation is
commonplace, the requirements are
now included within the main body of
the Regulations as it is no longer
considered to be a special location or
installation.
Overview
Some items of electrical equipment
are designed to have a current flowing
in the protective conductor when in
use; these currents are often created
by switch-mode power-supplies.
Commonly, IT (Information
Technology) or computer-processing
equipment use switch-mode powersupplies in applications where a
particular voltage is required for
operation.
Other items, such as electronic
ballasts in high-frequency fluorescent
luminaires or variable speed drives
Courtesy of Reddilight Limited
Individual items
Individual items of electrical
equipment having a protective
conductor current exceeding 3.5 mA
but not exceeding 10 mA shall be
either permanently connected to the
fixed wiring of the installation or
connected by means of a plug and
socket-outlet complying with BS EN
60309-2, and
Accumulative current
in the circuit
Where the accumulative protective
conductor current in a final or
distribution circuit is likely to
exceed 10 mA, a high integrity
connection shall be provided.
Levels of protective conductor current
The levels of expected protective
conductor current will vary depending
on the type of electrical equipment.
BS EN 60335-1 Household and similar
electrical appliances - Safety - Part 1:
General requirements states the
fundamental conditions to which
items of electrical equipment are
manufactured and, hence, should
perform when used under normal
operating conditions.
Clause 13.2 of BS EN 60335 states
that after the appliance has been
operated for a duration corresponding
to the most unfavourable conditions of
normal use, the leakage current
(protective conductor current) shall
IET Wiring Matters | Spring 08 | www.theiet.org
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6
not exceed the values shown in figure 1.
For terminology purposes, the term
"high" protective conductor current is
used when values of current exceed
3.5 mA flowing in the protective
conductor.
The dangers
Where a current flows in a protective
conductor, a voltage will present along
the length of the conductor and Ohm’s
law dictates the size of the potential
difference. If the connection to the
means of earthing is lost then the
metallic chassis of the electrical
equipment will rise in potential in
respect to Earth. Should a person or
livestock make contact with the
metallic chassis then current is likely
to flow resulting in an electric shock.
Class of equipment
All
Protective conductor
Portable, handheld,
movable, etc. equipment
Stationary
equipment
current (mA)
II
0.25
0
0.5
0I
0.5
III
0.5
I
0.75
I motor-operated
appliances
3.5
I heating appliances
0,75 mA, or 0,75 mA per kW
rated power input of the
appliance with a maximum of
5 mA, whichever is higher
Figure 1: Leakage currents as stated in Clause 13.2 of BS EN 60335
The requirements of 543.7
Regulation 543.7.1.1 requires that
individual items of electrical
equipment having a protective
conductor current above 3.5 mA but
not exceeding 10 mA should be either
permanently connected to the fixed
wiring, see figure 2, of the installation
or connected by means of a plug and
socket-outlet, see figure 3, complying
with BS EN 60309-2.
An earth monitoring system
to BS 4444
An earth monitoring system to BS 4444
(see figure 4) installed which, in the
event of a continuity fault occurring
in the protective conductor,
automatically disconnects the supply
to the equipment.
Figure 2: Example of a method of permanent
connection to fixed wiring - a fused
connection unit Courtesy of MK
Figure 3: Example of connection using
socket-outlet to BS EN 60309-2
Courtesy of MK
Proving unit
Protected equipment
A
Phase
D
The connection of equipment
The table shown in figure 5
(overleaf), shows the level of protective
conductor current with the
requirements of BS 7671:2008 for
equipment.
Final and distribution circuits
The table overleaf in figure 6 shows
the requirements of BS 7671:2008 for
every final circuit and distribution
IET Wiring Matters | Spring 08 | www.theiet.org
Neutral
CPC
Pilot
conductor
P
A = Alarm
A = Alarm
D = Relay contacts
D = Relay contacts
= Low voltage supply
V = Low voltageVsupply
S = Sensing device
S = Sensing device
V
Figure 4: Simplified circuit for an earth proving unit
S
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8
Regulation number
BS 7671:2008
Current
(mA)
Common rules
Chapter 51
0
543.7.1.1
3.5
The requirements of BS 7671:2008
No further requirements
Equipment shall be
543.7.1.2
10
Equipment shall be
permanently connected to
the installation
or
by means of a plug and
socket-outlet to
BS EN 60309-2
permanently connected to the installation, the
distribution and final
circuits shall be
connected by a flexible
cable with a plug and
socket-outlet to
BS EN 60309-2
or a single copper
conductor, 4 mm²
minimum with mechanical
protection
or
two individual CPCs
or
monitored by an earth monitoring device to
BS 4444
the CPC of the flexible cable not less than:
a single CPC 10 mm²
minimum
a protective conductor complying with Section 543
generally and monitored by
an earth monitoring device to
BS 4444 (see figure 4)
Where CPCs are part of a
multicore cable, the sum of
all conductors shall not be
less than 10 mm²
In the event of CPC
continuity failure, the
supply is automatically
disconnected
2.5 mm² for 16 A plugs
4 mm² for plugs 32 A and
above
the CPC of the flexible
cable not less than the line
conductor
In the event of CPC
continuity failure, the
supply is automatically
disconnected
Figure 5: Table showing the requirements of BS 7671 in relation to protective conductor current of equipment
circuit intended to supply one or more
items of equipment, such that the total
protective conductor current is likely
to exceed 10 mA:
Residual Current Devices
Regulation 411.3.3 requires that socket
outlets, not exceeding 20 A, that are for
use by ordinary persons intended for
general use, are protected by an RCD
rated at 30 mA or less, with an
operating time not exceeding 40 ms at
a residual current of 5 IΔn; an
exception is permitted if the socketoutlets are under the supervision of
skilled or instructed persons.
When designing such circuits, the
IET Wiring Matters | Spring 08 | www.theiet.org
designer must be aware of the type of
equipment likely to be supplied.
Accumulative high protective
conductor currents, in addition to
starting surges when many items are
energised simultaneously, can cause
the RCD to operate.
As there is no testing requirement
between 0.5 IΔn and IΔn, it is fair to
assume that a 30 mA RCD could
operate at any level of residual
current greater than 15 mA. Therefore,
it would be pertinent to design high
protective conductor currents for
any circuit protected by an RCD
rated at 30 mA at a level no greater
than 15 mA. See Guidance Note 3 for
more information on the testing
of RCDs.
Labelling at distribution boards
Circuits at distribution boards with
high protective conductor currents are
to be labelled accordingly so that
persons working on the distribution
boards can maintain the protective
precautions taken. The labelling
information is to be in accordance
with Regulation 543.7.1.5.
Where a circuit has or is likely to
have a high protective conductor
current, the protective conductor
connection arrangements at the
distribution board will be affected by,
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7/3/08
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Page 9
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Regulation
number
BS 7671:2008
543.7.1.3
The requirements
of BS 7671:2008
Current
(mA)
>10
TOP BRANDS
a single CPC 10
mm² minimum
The wiring of every
final circuit and
distribution circuit
intended to supply one
or more items of
equipment, such that
the total protective
conductor current is
likely to exceed
10 mA, shall have a
high integrity
protective connection
complying with one or
more of the following:
SAVE £100
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a single copper
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with mechanical
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QUOTE: 10442
249
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two individual Where CPCs are
part of a
CPCs
multicore cable,
the sum of all
or
conductors shall
not be less than
10 mm²
monitored by
an earth
monitoring
device to
BS 4444
(see figure 4)
In the event of
CPC continuity
failure, the
supply is
automatically
disconnected
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Contents: Slotted 0.4 x 2.5 x 80mm,
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0.5 x 3.0 x 70mm mains tester and rack.
QUOTE: 48111
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Figure 6: Table showing the requirements of BS 7671 in relation to the
protective conductor current of circuits
for example, the sequence of connections.
Regulation 543.7.1.4 requires that where two
protective conductors are utilised they are to be
terminated independently of each other at all
connection points, including accessories. This may
pose a problem at the earth bar of the distribution
board, i.e. there may not be an adequate number of
spare ways to terminate conductors separately. Some
large distribution boards may be provided with many
spare terminals on the earth bar, whilst some
manufacturers may supply an accessory kit to add
extra ways. Where this is not an option, the following
solution is one method of meeting the requirements:
The example is that the electrical installation has a
six-way distribution/consumer unit and consists of
six circuits:
Cable Rod Access Kit
Super Deluxe Set
•
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10
DB 1
1 Ring final circuit with high protective conductor current
2 Radial circuit with high protective conductor current
3 Ring final circuit
4 Radial circuit
5 Radial circuit
6 Radial circuit
Regulation 543.7.1.4 requires that where two protective
conductors are used on circuits with high protective conductor
currents, they are to be terminated into separate ways on the
earth bar.
By installing circuits 1 and 2 across two-ways of the earth bar,
in addition to marking each conductor with the correct circuitidentification ferrule, there is no ambiguity as to the function of
each conductor (see figure 7).
Regulation 543.7.1.5 requires that information is provided at the
distribution board indicating those circuits having a high
protective conductor current (see figure 8).
Figure 7: Termination of circuits with high protective conductor
currents at the earth bar
DB1
The following circuits have high
protective conductor current:
Further information
Guidance Note 1
Guidance Note 3
Guidance Note 5
Thanks to MK (mk.technical@honeywell.com) and
Reddilight Limited (info@reddilight.co.uk) for the images used. IET Wiring Matters | Spring 08 | www.theiet.org
1 Ring final circuit with high protective conductor currents
2 Radial circuit with high protective conductor currents
Figure 8: Information in accordance with Regulation 543.7.1.5
indicating those circuits with high protective conductor current